This application claims the benefit of U.S. Provisional Application No. 60/349,381, filed Jan. 22, 2002.
Commercially, fly ash is often used as a replacement for some of the Portland cement in concrete products. Surfactants, used to entrain air in the concrete mixtures, improve the workability of the mixtures and the durability of the concrete products to freeze-thaw cycles (Dodson, 1990).
Inefficient combustion and the use of low-NOx burners in coal-fired boilers have resulted in variable increases in the unburned carbon content of fly ash (Baltrus et al., 2001). When high-carbon fly ashes are used in concrete, an increase in the amount of surfactant may be required, because carbon absorbs the expectant surfactant in concrete mixtures. Variations in carbon content and amount of surfactant required directly impact the sale of fly ash for use with cement to produce concrete products. Even when fly ash meets loss on ignition (LOI) specifications, variation in the adsorption properties of the fly ash may result in changes in the amount of surfactant required (Freeman et al., 1997). The foam index (FI) test is used to determine the amount of surfactant required in the concrete. The FI test involves titration of a portion of the concrete mixture with an aqueous solution of surfactant until a stable foam results.
Factors affecting air entrainment in the concrete mixtures have been identified. For example, as the carbon content of the pozzolans increase, the level of entrained air decreases (Dodson, 1990). Freeman et al. (1997) examined the interactions of carbon-containing fly ash with surfactants and found that the interactions are time dependent and that the degree of interaction correlates only roughly with carbon content.
Gao et al. (1997) examined the interaction between several fly ash carbons and carbon blacks and an air-entraining admixture (AEA) and found that surfactant interaction increases with an increase in carbon surface area. Yu et al. (2000) found that a low specific area fly ash containing 17 wt % carbon produced by co-firing coal and petroleum coke had no measurable surfactant adsorptivity. Hill et. al. (1998) examined a number of fly ash samples using thermal and optical microscopy methods. They found that differential thermal analysis was not a useful prognostic tool for performance of fly ash in air-entrained mortar. Additionally, optical characterization of the forms of carbon in fly ash did not relate fly ash performance to mortar air entrainment, but it did indicate that a significant portion of carbon in fly ash is sub-micron in size. They also found that potential effects of carbon chemistry on surfactant adsorption capacity cannot be identified using surface areas determined with an inert gas such as nitrogen. Gao et al. (2001) reported ozonation for chemical modification of the carbon surfaces in fly ash as a route to reducing the adsorptivity of fly ash carbon toward surfactants.
One approach for characterization of fly ash carbon is to focus on partial oxidation to selectively remove each carbon form followed by characterization of the carbon form or forms remaining in the fly ash residues. LaCount et al. (1997), using this approach, characterized the carbon in several fly ash samples using a controlled-atmosphere programmed-temperature oxidation (CAPTO) instrument and found oxidation generally occurring in four different temperature zones. Several of the oxidation temperatures are well above those of coals, activated carbons, and other chars but significantly below the oxidation temperature of graphite. The amount of carbon dioxide evolving in each temperature range was evaluated. That work prompted progressive partial oxidation and pyrolysis studies of numerous fly ash samples followed by foam index (FI) measurements to assess any change in surfactant adsorption properties of each partially oxidized or pyrolyzed residue (LaCount et al., 1998 and 2001). A major decrease in FI occurred between room temperature and approximately 400° C. prior to significant loss of carbon.
Baltrus et al. (2001) optimized an ultraviolet-visible spectrophotometric method for measuring the adsorption of air-entraining surfactants on the components of cement. It was found that FI was a poor means for measuring adsorption capacity in high carbon fly ashes due to an insufficient equilibration time used in the foam index measurements.
A better understanding of the variation in interactions of air entraining surfactants with unburned carbon forms and the mineral components in fly ash concrete mixtures may lead to improved methodology for maintaining the level of air as the concrete cures. A better prediction of surfactant performance with different fly ash samples may help to minimize variability in concrete products.